BACTERIA AND MILK FERMENTATIONS 23 
patrons are expected to have milk as free as possible 
from these germs so that it may arrive at the creamery 
in a sweet condition. They are therefore expected to 
thoroughly cool and care for it, not alone to suppress 
the action of the lactic acid bacteria but also that of the 
abnormal species that might have gained access to the 
milk. 
While the acid bacteria are objectionable in milk, in 
cream made into butter they are indispensable. The 
highly desirable aroma in butter is the result of the 
growth of these organisms in the process of cream 
ripening. There are a number of different species of 
bacteria that have the power of producing lactic acid. 
2. CURDLING AND DIGESTING FERMENTATION. 
In point of numbers this class of bacteria ranks perhaps 
next to the lactic acid type. Indeed it is very difficult to 
obtain milk that does not contain them. It is not often, 
however, that their presence is noticeable owing to their 
inability to thrive in an acid medium. 
According to bacteriologists most of these bacteria 
secrete two enzymes, one of which has the power of 
curdling milk, the other of digesting it. The former 
has the power of rennet, the latter of trypsin. “As a 
rule,” says Russell, “any organism that possesses the 
digestive power, first causes a coagulation of the casein 
in a manner comparable to rennet.” 
It is only occasionally when the lactic acid organisms 
‘are in a great minority, or when for some reason their 
action has been suppressed, that this class of bacteria 
manfests itself by curdling milk while sweet. The curd 
thus formed differs from that produced by lactic acid in 
being soft and slimy. 
